The invention relates to methods and means for driving low pressure gas discharge lamps and, particularly, to flourescent lamps of that type. Commercial low pressure gas discharge lamps are basically low pressure mercury or sodium vapor discharge lamps designed to emit a maximum portion of energy of mercury or sodium spectrum. In the case of flourescent lamps this short wave ultraviolet energy is converted by the phosphor coating the inside of the tubes into visible light. A flourescent gas discharge lamp consists of a glass vessel, usually of tubular shape, coated inside with a phosphor powder. The vessel contains a mixture of one or more noble gases and a small amount of mercury vapor. To operate the lamp a gas discharge is maintained in it, mostly with the help of two electrodes with filaments at the ends of the tube. The discharge process is started and maintained with the aid of conventional magnet control gears or high frequency electronic control gears. Both types of control gears are used to convert the shape of electrical energy available from common AC or DC sources to the form necessary to start and hold up the discharge process. Generally, the energy delivered to the lamp is consumed by two main processes inside the lamp: discharge process and filament heating process. When energized, the state of gas discharge lamp from electrical point of view can be represented by the voltage across the lamp, discharge current flowing through the lamp and heating current flowing through the filaments.
In general, discharge lamps depending on gas mixture parameters as well as physical dimensions and shape will be characterized by different discharge voltage and current and will require different heating voltage. Thus, different control gears are used to drive respective discharge lamps having different ratings.
In a typical solution, the lamp is driven from an AC voltage source. Discharge and heating current is fixed to the value required by the sort of lamp and its constant.
In more advanced solutions, providing dimming of flourescent lamp, partially independent circuits are used to supply heaters of lamp as shown in U.S. Pat. No. 5,703,441. Heating voltage can be regulated while the lamp is entered into dimming mode providing optimal temperature of filaments. However, because discharge current is not regulated, the above solution can be used only to one type of lamp and can not be universal. Change of the lamp type requires changes in design.
Flourescent lamps even of the same size but manufactured by different producers and working under different climatic conditions require different starting voltages. At the same time power converters used to drive lamps have to provide high value of power factor that is equivalent to the resistive load of the mains. U.S. Pat. Nos. 4,870,327 and 4,958,108 are solving above mentioned problems but do not provide means for totally independent regulation of discharge current and heating current what is required for the gear capable to drive the lamp of any type. Proposed solution can be used only to the family of tubes having the same working point defined by discharge voltage and discharge current and varying in starting voltage.
Application of one type of electronic gear to different lamps requires automatic identification of the lamp type. The patent application EP 0413991 A1 attempts to solve that problem using starting voltage of the lamp to identify the type of lamp within a limited and predefined set of lamps. Starting voltage depends on many parameters like tube length and shape, gas mixture type and gas pressure and therefor can not be used as universal parameter identifying the lamp type. Identification algorithm proposed in the mentioned patent application allows only rough identification of the lamps within one limited family.
Fully universal identification of the lamp type can be achieved only through a measurement of all parameters defining electrically the lamp i.e.: discharge voltage, discharge current, heating voltage and heating current and appropriate identification algorithm taking into account all above mentioned parameters.
It is the principal object of this invention to provide a drive scheme for low-pressure gas discharge lamps providing independent supplying of discharge current and filament heating voltage from totally independent and regulated current and voltage sources in order to achieve automatic detection of the type of the lamp and automatic adjustment of discharge current and heating current to the values required by detected lamp.